To set up a connection between a mobile radio terminal and a base station in a mobile radio network, it is necessary to ascertain the time delay for the signal transmission between the two. This signal propagation time is required in a GSM system, for example, in order to synchronize the uplink bursts from the terminal to the timeslot pattern in the base station.
To this end, the base station determines the signal propagation time from the access bursts transmitted by the mobile station. A channel request message in the GSM system is sent as an access burst of this type. The base station then instructs the mobile station to bring forward its transmission time as appropriate by transmitting the determined signal propagation time to the mobile station in a channel allocation message, in the GSM system the “Immediate Assignment message”.
The signal propagation time is indicated in the GSM environment by the “timing advance value” (TA value). This is simultaneously a measure of the mobile station's distance from the respective base station. The TA value can thus be used as a basis for autolocation methods on the mobile station, for example.
The localization of a mobile radio terminal in a mobile radio network forms the prerequisite for providing the numerous “location services”. These are services which use different methods to make use of information regarding the position of the terminal or of the equipment user. Besides automatic transmission of the position or of the location in the case of an emergency call, they include position-dependent charging, position-dependent connection setup methods and position tracking, for example as part of traffic monitoring or fleet management.
One localization method is based on determining the cell in which the terminal is located. This provides actually usable statements in the core areas of large towns, in which the cell radii are in ranges of several hundred meters. In rural areas with their large cells, accuracies in the region of several kilometers are not adequate, however.
More accurate methods require expensive additional devices for the terminal. In the case of OTD (“Observed Time Difference”) methods, the mobile station has a complex design in order to determine the difference between the arrival times of particular signals that are emitted by a plurality of base stations. Also, a mobile station may be equipped with a GPS receiver, which increases the overall cost.
Other methods require additional devices in the mobile radio network, which likewise means high outlay. Thus, many ToA (“Time of Arrival”) methods are based on measuring the arrival times of a signal emitted by the mobile station at a plurality of known locations. This requires that what are known as LMUs (“Location Measurement Units”) be placed at numerous locations in the network.
By contrast, methods which use the signal propagation time between the terminal and the base station to determine the interval make it possible to dispense with expensive additional devices in the network. In this context, the mobile radio terminal is usually localized using three TA values ascertained by different base stations. However, these methods having drawbacks associated with their use.
One drawback is that the primary objective of the interchange of channel request and channel allocation messages outlined above is not to determine the signal propagation time and to transmit it to the mobile station, but rather to determine and reserve at least one radio channel for connection setup in the base station for the purpose of further interchange of messages. To this end, the channel allocation message is used to transmit the physical properties of the reserved channel (e.g. frequency, timeslot, subchannel number) to the mobile station.
A TA value is thus recorded in the base station in connection with the simultaneous setup of a connection via the air interface. The request for such a connection is setup with a reservation of at least one signaling channel (in the case of GSM: of a “Dedicated Control Channel”) and possibly of a further channel, e.g. a voice channel.
However, there is no requirement for channel reservation if only the signal propagation time is intended to be detected in the mobile station, such as for a terminal-aided localization method. Following receipt of the channel allocation method with the value it contains for the signal propagation time, the aim of the procedure has been achieved from the point of view of the mobile radio terminal. In a GSM system, the mobile station then changes to the RR idle state; the channel reserved by the base station is not required.
The reserved channel is released no earlier than after a few seconds, namely after the base station has detected that the reserved channel is not being used. In a GSM system, a typical value before release is six seconds. For this time, the requested channel is not available to other mobile stations for connection setup. This practice therefore wastes network resources. This problem has been an obstacle to acceptance of the inherently simple localization method described above to date, and hence to the use of localization methods on the whole.